It sounds like you have code in one of the events of the subform that
contributes this extra record.
Particularly, look out for anything in the Current event of the form, or its
AfterUpdate or AfterInsert events.
You can prevent the blank record from being accepted by opening the
subform's table in design view, and setting the Required property for one of
the fields to Yes. This might also give you a clue as to when the bad record
is being created.